What causes some vegetables and fruits to have hard white centers?

Stressful weather conditions, especially an up-and-down thermometer, disturb the ripening process. Fruit and vegetables exposed to low temperatures overnight are particularly susceptible to developing hard central cores. That’s one reason hard core centers are common earlier and later in the season while the weather is more changeable. Excessive fertilizer (especially accompanied by stressful weather) contributes to tough center core.

To prevent developing hard centers:

Provide warm, consistent temperatures during cool evenings by using frost protection, such as tunnel row covers, floating row covers, water barriers, or individual plant covers. Apply fertilizer in moderation, particularly during the first month that plants are in the ground.